Formation mechanism of Macrosegregation in reverse squeeze casting

The segregation behavior of impurity elements needs to be analyzed from the aspects of solidification thermodynamics and kinetics. From the thermodynamics point of view, as long as the equilibrium distribution coefficient of solid solution formed with Al is not equal to 1, there is solute redistribution in the solidification process, and the extent of this redistribution can be quantitatively described by the value of equilibrium distribution coefficient. The equilibrium distribution coefficients (K0) between impurity elements and Al in A356 aluminum alloy are shown in Table 6. Cu, CA, Fe and Mn all form solid solution with Al and eutectic reaction occurs. Therefore, their equilibrium distribution coefficients (K0) are less than 1; However, peritectic reaction occurs between Zr and Al, and the equilibrium constant K0 is greater than 1; Pb and Al do not form solid solution and compound, and monotectic reaction occurs, and there is no equilibrium distribution coefficient. Therefore, the impurity elements can be divided into three categories according to the equilibrium partition coefficient: easy segregation elements, difficult segregation elements and abnormal segregation elements. The equilibrium partition coefficient of Mn is close to 1, so the segregation tendency of Mn is the smallest and it is not easy to segregate. On the contrary, the equilibrium partition coefficients of Ca, Fe, Zr and Cu are far away from 1, so they tend to segregate easily; There is no equilibrium distribution coefficient between Pb and Al, which belongs to abnormal segregation element.

However, the largest segregation ratio of impurity elements in the test results is not Ca and Fe, but Zn, P, Cu and Pb. In addition, the order of longitudinal and radial segregation ratio is not the same, which indicates that the segregation degree of impurity elements in backward squeeze casting is controlled not only by equilibrium distribution coefficient, but also by rheological behavior.

(a) Filling (b) Pressure holding solidification (c) Rheological feeding

Under the condition of squeeze casting, when the pressure is up to 60-150mpa, not only the residual liquid phase can flow, but also the solid-liquid mixture can flow, and even the solidification layer of aluminum alloy can have plastic flow. This flow causes the macro transfer of material and changes the distribution of impurity elements. The process of reverse squeeze casting wheel hub can be divided into three stages: extrusion filling, pressure holding solidification and rheological feeding. In the extrusion filling stage, the liquid Al has just been poured into the mold cavity, and a very thin solidification layer is formed only at the parts contacting with the mold wall (the bottom, the lower part of the side and the contact surface of the indenter). The rest part is a liquid with basically uniform temperature and solute content. Therefore, although the rheologic distance of the aluminum alloy is the largest in the filling stage, there is no inhomogeneous composition; It is a coupling stage that solidification and rheology occur at the same time. Under the action of high pressure, the interface heat transfer intensifies, the solidification rate increases greatly, and the solute redistribution is difficult to reach equilibrium. Therefore, there will be a distribution gradient of impurity elements in the solidification layer, and the enrichment of impurity atoms (K0 < 1 impurity) or the depletion of impurity atoms (K0 > 1 impurity, such as Zr) appear in the solidification front. As the solidification proceeds, the condensed solid shrinks. Under the action of negative pressure and indenter pressure produced by the shrinkage, the impurity enrichment or depletion area in the solidification front will flow to the shrinkage part along the feeding direction. This flow makes the solute enrichment layer transfer to the shrinkage part, and the solute content in the solidification front decreases, which accelerates the solidification process and discharges the solute again. Therefore, the melt rich in impurity elements is continuously transported and transferred to the shrinkage zone of the distal end. With the extrusion solidification, the rheological feeding channel becomes narrower and narrower, so that the shear stress required for the rheological transfer of solute rich melt (solid-liquid mixture) in the channel becomes larger and larger. Once the applied shear stress is less than the critical shear stress, the channel will be frozen, forming a “channel segregation” rich in impurity elements.